Such a data communication system is known in a mobile radio installation, for example an amateur radio mounted in a car. In a mobile radio installation, there are many pieces of radio communication equipment which are so arranged as to be suitable for mounting in a vehicle. The radio communication installation is separated into a main device which has built therein transmitting and receiving functions and a display/operating device (hereinafter referred to as "operating device") which controls operation of the main device. The operating device is mounted in the limited driver space and is connected to the main device through a cable. A microphone for communication and switches for controlling the main device are mounted in the operating device. This makes it necessary to send and receive data through the cable.
The main device of the radio communication equipment is provided with a master or main microcomputer while the operating device is provided with a slave or sub-microcomputer. The arrangement is such that required computation is carried out by each microcomputer, whereby data are transmitted between the main device and the operating device through a cable. The transmission is carried out serially under control of a clock signal. The transmission is based on a signal with a square waveform, such that a particular level of the signal corresponds to the presence of a bit. The cable connecting the two microcomputers has an internal capacitance, because of which the waveform of the signal is deformed. This deformation makes it difficult to precisely detect the presence of bits and introduces a risk for transmission errors. A way to improve the detection would be to lower the frequency of the clock signal but this reduces the data transmission rate.